Many of the mechanisms of self-renewal and differentiation of hematopoietic precursor cells, including the regulation of these processes, remain to be elucidated. In many respects, further detailed experimental analysis of hematopoiesis depends on the identification, isolation, and molecular characterization of pluripotent and committed hematopoietic precursor cells. Immunologic identification and study of subsets of cells has led to dramatic progress in understandng other differentiating organ systems; and initial studies (including our own) reveal impressive heterogeneity of myeloid cell has led to dramatic progress in understanding other differentiating organ systems; and initial studies (including our own) reveal impressive heterogeneity of myeloid cell surfaces that should allow manipulation by immunologic probes. We propose further development and use of murine and human monoclonal antibodies, specifically directed against small subsets of myeloid cells, to approach the identification and isolation of human hematopoietic precursor cells. Several collectively unique approaches to antibody development and characterization will be employed. Resulting antibodies will be used to isolate precursor cells, and to study hematopoiesis in model systems. This work might have eventual broad application for the understanding, diagnosis and treatment of leukemia and aplastic anemia in man.